Controlling ragweed pollen in Detroit: A no-mow solution for Motown?

http://www.eurekalert.org/pub_releases/2014-06/uom-crp061614.php

PUBLIC RELEASE DATE: 16-Jun-2014
Contact: Jim Erickson
University of Michigan
Controlling ragweed pollen in Detroit: A no-mow solution for Motown?

ANN ARBOR—When it comes to controlling hay fever-triggering ragweed plants on Detroit vacant lots, occasional mowing is worse than no mowing at all, and promoting reforestation might be the best solution.

Those are the findings of a new University of Michigan study that surveyed vacant lots in several Detroit neighborhoods for ragweed, counting the number of ragweed plants and estimating how often each lot was mowed.

The researchers found that ragweed was significantly more likely to be present in vacant lots mowed once a year or once every two years—a common practice in Detroit, which has one of the highest proportions of vacant lots in the United States—than in lots mowed monthly or not at all.

"When we surveyed vacant lots, we found that some mowing is worse than no mowing," said Daniel Katz, a doctoral candidate at the U-M School of Natural Resources and Environment. "This is because occasional mowing, say once a year or once every other year, creates the disturbed conditions in which ragweed plants thrive."

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Based on the findings of the two studies, the authors recommend that vacant lots in Detroit either be mowed regularly or not at all. A report issued May 27 by the Detroit Blight Removal Task Force states there are 114,033 vacant lots in the city.

"Although allowing vacant lots to reforest is controversial, it is already happening in many places across Detroit. Woody plants are establishing in vacant lots and reclaiming large chunks of Detroit," Katz said. "Regardless of whether people think that reforestation of vacant lots is a good or bad thing overall, it will have the benefit of reducing ragweed pollen exposure."

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In Detroit, vacant lots are most common in low-income neighborhoods that have large minority populations. Katz and Carey say the results of their land-use study suggest that local production of ragweed pollen disproportionately affects such communities, which raises questions about environmental justice.

The U-M studies found that vacant lots are the main habitat for ragweed in Detroit: Ragweed densities were six times higher in vacant lots than at locations around occupied homes in the city.

Vacant lots contained up to 42 ragweed plants per square meter. When you consider that the tiny, drab flowers of a single ragweed plant can produce up to a billion pollen grains annually, it's clear that Detroit vacant lots are pollen factories churning out a noxious product that afflicts thousands of city residents.

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They found that 28 percent of the unmowed lots contained ragweed plants, while 63 percent of the annually mowed lots and 70 percent of the biennially mowed lots contained the weed. Vacant lots mowed at least once a month during the growing season had no ragweed plants.

"When these lots are left alone completely, other plants rapidly outcompete ragweed," Katz said.

When the lots are left unmowed, the common perennial plants that replace ragweed include goldenrod, milkweed, Kentucky bluegrass, chicory and aster. When a vacant lot is not mowed for several years, the young trees that can encroach include the Norway maple, silver maple, box elder, cottonwood and tree of heaven.

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